Instead of a widely spreading optical transmission system whose transmission rate is 2.5 Gbit/s or 10 Gbit/s, an ultra-high-speed long-distance optical transmission system with 40 Gbit/s, 100 Gbit/s or more has been put to a practical use. In the ultra-high-speed long-distance optical transmission system, a coherent transmission system has been viewed as important instead of an amplitude modulation-direct detection receiving system. In the optical transmission system employing the coherent transmission system, an optical phase modulation technology is used in an optical transmitter. A digital coherent reception technology in which a coherent reception technology and a digital signal processing technology are combined is used for an optical receiver. An optical phase modulation system excels in the characteristics such as an optical signal to noise resistance characteristic, a wavelength dispersion resistance characteristic, and a polarization mode dispersion resistance characteristic which are requested for a long-distance optical fiber transmission.
Research and development on the optical phase modulation system, particularly a modulation and demodulation system such as binary phase shift keying (BPSK), quadrature PSK (QPSK) and polarization multiplexing-QPSK (PM-QPSK) is widely performed from a view point of balance among a transmission characteristic, feasibility and cost. In PM-QPSK, 4 values phase modulation signals which have superiority in optical frequency utilization efficiency are multiplexed by two orthogonal polarized waves in order to expand a transmission capacity without increasing an optical frequency bandwidth to be used.
In the long-distance optical multiplex relaying transmission which employs an optical amplifier, a parameter which has the largest impact on a reception characteristic of a signal light is the optical signal to noise ratio (OSNR) regardless of modulation and demodulation systems. As the transmission distance gets longer, signal degradation caused by a nonlinear optical effect of an optical fiber provides the stronger influence in addition to the signal degradation caused by OSNR deterioration. When performance of a transmission apparatus is monitored, it is very important to specify reception characteristics and deterioration cause of a signal light received in an optical receiver.
In relation to the present invention, Patent Literature 1 (Japanese Patent Application Laid-Open No. 2009-232082) discloses the configuration in which the dispersion compensator is controlled based on the result on identification whether or not the factor of signal quality deterioration is caused by the dispersion in the optical signal transmission apparatus.